In a tablet display device, thin film transistors (TFTs) arranged in matrix and a driving chip for driving the thin film transistors are generally provided on an array substrate, in order to control every pixel. In general, the thin film transistors are formed in a display area by a patterning process, and the driving chip is bonded in a non-display area.
With the development of display technology, GOA (Gate on Array) technique has been developed, that is, a gate driving circuit in the array substrate is integrated on a base substrate to drive pixels in a scanning manner. The gate driving circuit is formed by thin film transistors, and can be formed by a patterning process like the thin film transistors in the display area. Compared with the traditional bonding process of the driving chip, the bonding with respect to the GOA chip (integrated gate driving chip) is omitted, which can not only save cost, but also facilitate improving capacity due to the omission of bonding process in the gate line direction, and thus the GOA technique is widely applied in thin film transistors in the non-display area of the tablet display device.
A thin film transistor generally includes a gate G, a source S, a drain D and a semiconductor layer. Currently, the structure of the GOA thin film transistor in the non-display area differs from that of the pixel thin film transistor in the display area in that: a trench between the source and the drain of the GOA thin film transistor shown in FIG. 1 generally has a U-shaped structure, whereas a trench between the source and the drain of the pixel thin film transistor shown in FIG. 2 generally has a linear shape. In comparison, the trench between the source and the drain of the GOA thin film transistor is more space-saving, and is thus suitable for the non-display areas having limited space.
However, in forming the array substrate by a patterning process, in an exposing step, the entire substrate is exposed in a same exposure direction (X direction or Y direction according to the arrangement of the thin film transistors on the substrate), since the trench between the source and the drain of the GOA thin film transistor has a different shape from that of the trench between the source and the drain of the pixel thin film transistor, exposure in the GOA area and exposure in the pixel area are likely to be different (e.g., non-uniformity, some region in a trench is well exposed while another region is poorly exposed); accordingly, photoresist residues remain in a trench region between the source and the drain due to poor exposure in a developing step subsequent to the exposing step; as a result, incomplete etching of a metal film of the source and the drain occurs in the trench region between the source and the drain in the GOA area or the pixel area due to photoresist residues. Non-uniform exposure in any one of the GOA area and the pixel area will influence the characteristics of the thin film transistors, and influence the display quality of the product.
It can be seen that, it has become an urgent technical problem to standardize structures of thin film transistors in the non-display area and the display area so as to ensure that the thin film transistors having different structures in the above two areas have a same performance for exposing the trenches between the sources and the drains thereof during an patterning process.